Hi - exTA (male) here :
After I retired from ten years as TA in an infant school, then two years in a comprehensive, I did several years as a voluntary helper in a small primary school with only three classes, and I helped in the R/Yr1/Yr2 class.
The teacher was very good and well organised, and there was certainly no disadvantage to any child due to the mixed ages. Indeed, for the younger, able child there is the advantage of sometimes working with a higher group; and for the older but less able, they might sometimes work at a lower, less stressful, level. A good teacher should be differentiating lessons within a class anyway, and even with three different ages in a class the spread of ability might not be very much different from the range of abilities within a single year group class.
So I don't think you need have any worries for DD whatsoever, PROVIDED that the teacher is competent and organised; as you consider it a good school, then one assumes she should be.
DD might not thank you for the 'giant' label (!), but she should be happy to maybe be similar in size to some older Yr1s.
Also, please don't fret TOO much over academic performance in a primary school, as many other aspects are also important : ethos; environment; the rest of the curriculum, particularly physical activities, arts & crafts, singing & music; clubs, outside visits, links with other schools, or the wider community.
[ Glue your hair back on, and no more anxiety, please! ]